One thing to note is that the 24-105/4L IS has a more effective IS system than the 28-135/3.5-5.6 IS. The 28-135 was one of Canon's first IS lenses and Canon claim about 2 stops of added stability. The 24-105 has a newer IS system which Canon claim adds about 3 stops. When you take into account the f4 vs. f5.6 , this gives you the ability to shoot in about 2 stops lower light at the telepho end of the 24-105 than would be possible with the 28-135.
I have both lenses and I'd say that the 24-105 shows a little better contrast then the 28-135 and the chromatic aberration is lower (though there is still some). It seems sharper, especially in the mid range. The ability to zoom to 24mm rather than switch lenses is also nice. There isn't a "night and day" difference between the lenses, but objectively tested, the 24-105 is a little sharper and with the faster speed, more effective IS and ability to zoom to 24mm I though it was a worthwhile buy, especially for the $700 price tag when bought as a kit with the 5D (which saves you about $350). I might not have spent $1050 for one, but at $700 I think it's worth it. Even if for some reason you later decided that the 28-135 was good enough, I'm pretty sure you could easily get back the $700 you'd have paid for the lens.
I bought my
EOS 5D + 24-105L kit ( $2599.99 with free shipping) from Amazon when the instant rebates started and I'm pretty happy with both of them.
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